Oscillators are electronic circuits that generally convert energy into a periodic signal. Oscillators often provide repeating constant or highly regular frequency signals such as clock signals. The output of the oscillator can be a variety of waveforms including sinusoidal, triangular or square waves. The quality of an oscillator is described by the duty cycle, thermal stability and more recently the power consumption. Duty cycle relates to the amount of time a signal is above a threshold. Thermal stability indicates how much frequency drift an oscillator experiences across its operating temperature range. Power consumption is part of power management and in the current era of battery powered mobile devices, therefore power consumption is of significant importance in an oscillator. The oscillators tend to run continuously, consuming power even when other parts of a circuit are in a sleep mode, making power consumption in oscillators critical to power reduction in a circuit.
Oscillators include at least two types: tuned oscillators and relaxation oscillators. While tuned oscillators have their uses in high power and high precision applications, the relaxation oscillator circuit is a good candidate for oscillators in an integrated circuit because a relaxation oscillator requires little silicon area, can be completely fabricated on an integrated circuit, and requires neither a crystal nor an inductor to operate.
A relaxation oscillator works by charging a capacitor to a threshold voltage VT. When the capacitor charges to the voltage VT the capacitor discharges, and the process repeats. The capacitor typically charges from a DC voltage supply coupled through a resistor. The RC time constant determines the frequency of the relaxation oscillator output. A simple relaxation oscillator that uses an RC network to determine its frequency can be prone to frequency drift, because the resistor and capacitor values vary with temperature. Components that are commonly available in integrated circuit processes are positive temperature coefficient resistors (RTP) and negative temperature coefficient resistors (RTN). By using an RTN resistor and an RTP resistor in series or parallel, the temperature deviation of the overall resistance value can be made more stable across a temperature range.
FIG. 1 shows a block diagram of a conventional relaxation oscillator. The low power oscillator (LPO) 100 is a relaxation design that uses capacitor 104 and resistor 108 to create a ramping voltage. Comparator 110 triggers at a voltage on node 115, and the output of the comparator is coupled to a pair of NAND gates and to a final inverting buffer 120. The comparator reference voltage of node 115 is set by the values of resistors R3 and R2, based on the unbuffered LPO output. The power on switch (PWD) 102 is normally open, allowing the LPO to oscillate. In the open state, the pair of NAND gates operate as inverters. While PWD is asserted, the NAND gate outputs both go low, forcing the final output of the LPO to go to a constant high due to the inverting buffer 120.
In the relaxation oscillator 100, the comparator 110 provides a reliable trigger based on the ratio of resistors R2 and R3. When resistors R2 and R3 are located in close proximity to one another on an integrated circuit, the ratio can track well over process and temperature variations. The stability of the RC time constant of the ramping voltage can be further improved by making the resistors from a pair of resistors in parallel, a first resistor as a RTN and the second resistor as a RTP resistor, which are commonly available in various semiconductor fabrication processes. The delay time from a trigger event to the comparator output change also varies with power and temperature. Increasing power can shorten the comparator delay (reduce latency) and reduces dependency on temperature. However, this approach increases power consumption. Oscillators with improved thermal stability and with reduced current consumption are needed.